Engine Control Module: Description and Operation

Powertrain Control Module (PCM):

PURPOSE
The "Control Module" (referred to as the PCM - Powertrain Control Module) is located in the passenger compartment and is the control center of the Computer Command Control (C3) system. The PCM constantly monitors various sensors, compares this information with programmed information stored in its memory, and operates output devices to control fuel delivery, idle speed, ignition timing, and certain emission control systems.


OPERATION
The PCM has several serviceable parts. A controller, called a PCM, and one of the following combinations of programming memory; a PROM (Programmable Read-Only Memory), a PROM and a CAL-PAK (Calibration Package), or a MEM-CAL (Memory Calibration unit), which also contains the fail-safe programming to operate the vehicle if the PCM or other vital system fails.

INPUTS AND OUTPUTS
The following is a list of some of the systems controlled by the PCM.

^ Fuel Delivery (pump) and Fuel Injection System
^ Ignition / Electronic Spark Timing (EST)
^ Electronic Spark Control (ESC - knock sensor)
^ Air Conditioning Compressor Clutch
^ Coolant Fan
^ Idle Speed Control
^ Evaporative Emission Control System (purge control)
^ Torque Converter Clutch. (TCC)
^ Electronically Controlled Automatic Transmission (shift control)

The computer controls output devices through Quad Driver Modules (QDM). Most output devices are operated by completing the circuit to ground. When the PCM is commanding a device or a component "ON," the quad driver for that device completes that circuit to ground. When the PCM is commanding a device or component "OFF," the quad driver opens the ground circuit. Inputs may be related to more than one output. If an input device fails, such as the oxygen sensor, this could affect more than one of the systems controlled by the computer.


SELF DIAGNOSTIC FUNCTION
The computer can recognize problems with sensors, output devices and circuits, and alert the driver by turning on the "SERVICE ENGINE SOON" light on the instrument panel. Problems in specific devices or circuits are identified by "trouble codes", which are stored in the computers memory when a problem is detected in a computer controlled device or circuit. Trouble codes can be read using a "SCAN" tool or by entering the PCM into diagnostic mode and counting the flashes of the SES light. See DIAGNOSIS, TESTING AND INSPECTION PROCEDURES/TESTING PROCEDURES/ON-BOARD DIAGNOSTICS for display procedures and trouble code descriptions.

ADAPTABILITY
The system has a learning ability which allows it to make corrections for minor variations in the fuel system to improve driveability. The computer stores actual sensor signal values in its memory during normal operation and uses them as the default values when the engine is started again. This ensures good performance and driveability each time the vehicle is driven, even if conditions are changing. If the battery is disconnected to clear trouble codes or for repair, the learning process has to begin all over again, and a change may be noticed in the driving performance of the vehicle. To reset the vehicles learning ability, make sure the engine is at operating temperature and operate the vehicle at part throttle, moderate acceleration, and idle conditions, until normal performance returns.

FAIL-SAFE OPERATION
The system also has a built in back-up system to provide near normal driving conditions during system malfunction. When problems are detected in certain systems, the PCM will revert to its fail-safe mode so the vehicle can be driven until repaired.